Many modern electronic systems require more than one operating voltage. New generation integrated circuits, for example, call for multiple power supply voltages that are under 5 volts direct current (DC). Devices such as logic gates or microprocessors commonly require voltage sources of 5 V/3V3, 3V3/2V9, or 3V3/1V5. Thus, there is a demand for power conversion products capable of providing multiple, well regulated, low output voltages in a low cost package.
An example of a forward converter that produces multiple output voltages may be seen in FIG. 1. FIG. 1 is a circuit diagram of an isolated forward converter 2 with dual output voltages V.sub.out(1) and V.sub.out(2). Converter 2 includes a conventional isolated forward converter, which produces V.sub.out(1) across load resistor 20, and an additional voltage output source, which produces V.sub.out(2) across load resistor 36. The conventional isolated forward converter of converter 2 includes a transformer 4, a transistor 6 and capacitor 8, a diode 10, a free wheeling diode 12, an inductor 14, a capacitor 16, and a control circuit (not shown). Control of a conventional isolated forward converter is well known in the art. An explanation of the operation of the conventional isolated forward converter of converter 2 is therefore omitted for brevity.
To generate output voltage V.sub.out(2), converter 2 has a second inductor 24 and second capacitor 26 connected to transformer 4 via a magnetic amplifier 28, and a diode 30. A second free wheeling diode 32 is also connected between second inductor 24 and ground potential. Thus, inductor 24 is connected to ground potential via a separate free wheeling path than used by inductor 14. A magnetic amplifier control circuit 34 causes magnetic amplifier 28 to block and unblock the flow of current from transformer 4 into second inductor 24.
Converter 2 suffers power losses because of the wire gauge and number of turns in magnetic amplifier 28. Because magnetic amplifier 28 is driven into saturation each cycle, magnetic core losses can be high. These losses are worsened at high switching frequencies used to achieve high packaging densities in DC to DC converters. Additionally, converter 2 can be expensive due to the cost of magnetic amplifier 28 and the number of switching devices required for operation of converter 2, i.e., diodes 10, 12, 30, and 32.
FIG. 2 is a circuit diagram of another isolated forward converter 50 having dual output voltages. Converter 50 is similar to converter 2, like-numbered elements being the same. However, in converter 50 inductor 24 is connected, via a transistor 52, to a node between diode 10 and inductor 14. Converter 50 uses an electronic magnetic amplifier 51, which includes transistor 52 controlled by a control circuit 54. For example, control circuit 54 may be model number CS5101 manufactured by Cherry Semiconductor, Inc. As with converter 2, converter 50 has separate free wheeling paths for inductors 14 and 24, i.e., via free wheeling diodes 12 and 32, respectively. However, unlike converter 2, in converter 50 current flows from transformer 4 to both inductor 14 and inductor 24 via diode 10.
Circuit 50 requires a complicated bias voltage and gate drive voltage implementation for transistor 52. Further, there are potential cross regulation problems because transistor 52 is off when diode 10 is back biased. During the free wheeling period, there is no communication between inductor 14 and inductor 24. Thus, where V.sub.out(2) is loaded and V.sub.out(1) is not, converter 50 will be underpowered due to the lack of a path for current to flow between V.sub.out(1) to V.sub.out(2). Consequently, a load must be maintained on V.sub.out(1). Converter 50 is also expensive because of the number of switching devices, i.e., diodes 10, 12, 32, and transistor 52, and because of the expense associated with electronic magnetic amplifier 54.